DNS query

The resolver class is used to do DNS query, i.e., convert a host+service to IP+port. Take boost::asio::ip::tcp::resolver as an example:

#include <boost/asio.hpp>
#include <iostream>

int main()
{
    try
    {
        boost::asio::io_context io_context;

        boost::asio::ip::tcp::resolver resolver{io_context};
        boost::asio::ip::tcp::resolver::results_type endpoints =
                resolver.resolve("google.com", "https");

        for (auto it = endpoints.cbegin(); it != endpoints.cend(); it++)
        {
            boost::asio::ip::tcp::endpoint endpoint = *it;
            std::cout << endpoint << '\n';
        }
    }
    catch (std::exception& e)
    {
        std::cerr << e.what() << '\n';
    }

    return 0;
}

The running result is:

74.125.24.101:443
74.125.24.139:443
74.125.24.138:443
74.125.24.102:443
74.125.24.100:443
74.125.24.113:443

The element of boost::asio::ip::tcp::resolver::results_type's every iteration is basic_resolver_entry:

template <typename InternetProtocol>
class basic_resolver_entry
{
......
public:
  /// The protocol type associated with the endpoint entry.
  typedef InternetProtocol protocol_type;

  /// The endpoint type associated with the endpoint entry.
  typedef typename InternetProtocol::endpoint endpoint_type;
......
  /// Convert to the endpoint associated with the entry.
  operator endpoint_type() const
  {
    return endpoint_;
  }
......
}

Since it has endpoint_type() operator, it can be converted to endpoint directly:

boost::asio::ip::tcp::endpoint endpoint = *it;